The invention relates to an apparatus for the pretreatment and subsequent plastification or agglomeration of plastics.
The prior art reveals numerous similar apparatuses of varying design, comprising a receiver or cutter compactor for the comminution, heating, softening and treatment of a plastics material to be recycled, and also, attached thereto, a conveyor or extruder for the melting of the material thus prepared. The aim here is to obtain a final product of the highest possible quality, mostly in the form of pellets.
By way of example, EP 123 771 or EP 303 929 describe apparatuses with a receiver and, attached thereto, an extruder, where the plastics material introduced into the receiver is comminuted through rotation of the comminution and mixing implements and is fluidized, and is simultaneously heated by the energy introduced. A mixture with sufficiently good thermal homogeneity is thus formed. This mixture is discharged after an appropriate residence time from the receiver into the screw-based extruder, and is conveyed and, during this process, plastified or melted. The arrangement here has the screw-based extruder approximately at the level of the comminution implements. The softened plastics particles are thus actively forced or stuffed into the extruder by the mixing implements.
Most of these designs, which have been known for a long time, are unsatisfactory in respect of the quality of the treated plastics material obtained at the outgoing end of the screw, and/or in respect of the quantitative output of the screw. Studies have shown that the requirements placed upon the screw downstream of the container, mostly a plastifying screw, differ during the course of the operation.
For material that is thermally and mechanically homogeneous, there is usually a quality improvement in the product obtained at the outgoing end of the screw when the flight depth of the metering zone of the screw is very large and the screw rotation rate is kept very small. Studies have shown that the reason for this is that this type of screw geometry subjects the processed material to a low level of shear. The shear level to which the processed product is exposed (shear rate) is calculated from the circumferential velocity of the screw, divided by the flight depth of the screw. This type of screw geometry subjects the material only to a low level of mechanical and thermal stress.
However, if it is desirable to increase the quantitative output of the screw or to improve the performance for example of a shredder-extruder combination, the screw rotation rate must then be raised, and this means that the shear level is also raised. However, this causes the screw to subject the processed material to higher mechanical and thermal stress.
However, an effect that occurs both with slow-running and deep-cut screws having large flight depth and with fast-running screws is that, as previously mentioned, differences in quality of individual batches of material introduced to the screw, e.g. differences in flake size and/or differences in temperature of the plastics material, have a disadvantageous effect with regard to inhomogeneity of the plastics material obtained at the outgoing end of the screw. In order to compensate for this inhomogeneity, the temperature profile of the extruder is in practice raised, and this means that additional energy has to be introduced into the plastic, thus subjecting the plastics material to the thermal damage mentioned and increasing the amount of energy required. Another result here is that the viscosity of the plastics material obtained at the outgoing end of the extruder is reduced, and this makes the material more free-flowing, with concomitant difficulties in the further processing of this material.
It can be seen from this that the process parameters that are advantageous for obtaining material of good quality at the outgoing end of the screw are mutually contradictory.
The fundamental task of the extruder screw is intake, conveying, and melting or agglomeration of the plastics material and then homogenization of the same. For this purpose, it has to generate a certain pressure.
A traditional extruder screw with constantly increasing root diameter is fundamentally divided into three functional regions. This type of three-zone screw is the most commonly used screw type, which can process very many types of material. In the intake zone, the material is drawn into the region of the screw and conveyed onward through the rotation of the screw. In the compression zone, the material is compacted by virtue of the decreasing flight depth, and melted or agglomerated. In the metering zone, the melt or the agglomerate is brought to the desired processing temperature and homogenized and fully melted. The necessary pressure is moreover generated in order to overcome the resistance of the die. This has an effect on the throughput rate.
Factors of substantial importance for the melting behaviour or agglomeration behaviour of the pretreated or softened polymer material passing from the cutter compacter into the extruder, and for the product quality finally obtained, and subsequently for the throughput rate or quantitative output rate of the extruder are accordingly inter alia the length of the individual regions or zones, and also the parameters of the screw, e.g. its thickness, flight depths, etc.
However, particular conditions are present in the present cutter compacter-extruder combinations, since the material which passes into the extruder is not introduced directly, untreated and cold but instead has already been pretreated in the cutter compacter, i.e. heated, softened and/or partially crystallized, etc. This is among the decisive factors for the way in which the extrusion process proceeds, and for the final quality of the melt or of the final products.
The two systems, i.e. the cutter compacter and the extruder, have an effect on each other, and the results of the extrusion process are greatly dependent on the pretreatment, just as the extrusion process can compensate for, and have an effect on, certain parameters of the pretreatment process.
The interface between the cutter compacter and the extruder, i.e. the region where the pretreated material is transferred from the cutter compacter into the extruder, is therefore an important region. Firstly, this is a purely mechanical problem point, since two apparatuses that operate differently must be coupled to one another here. This interface can moreover also pose problems for the polymer material, since the material here is mostly in a greatly softened condition, close to the melting range, but is not permitted to melt. If the temperature is too low, the throughput and the quality fall, but if the temperature is too high and undesired melting occurs at some points, the intake becomes blocked.
It is moreover difficult to achieve precision of metering and feed into the extruder, since a closed system is involved and there is no direct access to the intake, but instead the material is fed into the extruder from the cutter compacter, and the feed cannot therefore be influenced directly, for example by way of a gravimetric metering system.
It is therefore of decisive importance that mechanical aspects of this transition are carefully designed, i.e. involving an understanding of the properties of the polymer, and that, at the same time, the cost-effectiveness of the entire process is taken into account, i.e. high throughput and appropriate quality. Some of the preconditions that require attention here are contradictory.
Another feature shared by the apparatuses known from the prior art and mentioned in the introduction is that the direction of conveying or of rotation of the mixing and comminution implements, and therefore the direction in which the particles of material circulate in the receiver, and the direction of conveying of the extruder, are in essence identical or have the same sense. This arrangement, selected intentionally, was the result of the desire to maximize stuffing of the material into the screw, or to force-feed the screw. This concept of stuffing the particles into the conveying screw or extruder screw in the direction of conveying of the screw was also very obvious and was in line with the familiar thinking of the person skilled in the art, since it means that the particles do not have to reverse their direction of movement and there is therefore no need to exert any additional force for the change of direction. An objective here, and in further derivative developments, was always to maximize screw fill and to amplify this stuffing effect. By way of example, attempts have also been made to extend the intake region of the extruder in the manner of a cone or to curve the comminution implements in the shape of a sickle, so that these can act like a trowel in feeding the softened material into the screw. Displacement of the extruder, on the inflow side, from a radial position to a tangential position in relation to the container further amplified the stuffing effect, and increased the force with which the plastics material from the circulating implement was conveyed or forced into the extruder.
Apparatuses of this type are in principle capable of functioning, and they operate satisfactorily, although with recurring problems:
By way of example, an effect repeatedly observed with materials with low energy content, e.g. PET fibres or PET foils, or with materials which at a low temperature become sticky or soft, e.g. polylactic acid (PLA) is that when, intentionally, stuffing of the plastics material into the intake region of the extruder, under pressure, is achieved by components moving in the same sense, this leads to premature melting of the material immediately after, or else in, the intake region of the extruder. This firstly reduces the conveying effect of the extruder, and secondly there can also be some reverse flow of this melt into the region of the cutter compactor or receiver, with the result that flakes that have not yet melted adhere to the melt, and in turn the melt thus cools and to some extent solidifies, with resultant formation of a clump or conglomerate made of to some extent solidified melt and of solid plastics particles. This causes blockage on the intake of the extruder and caking of the mixing and comminution implements. A further consequence is reduction of the throughput of the extruder, since adequate filling of the screw is no longer achieved. Another possibility here is that movement of the mixing and comminution implements is prevented. In such cases, the system normally has to be shut down and thoroughly cleaned.
Problems also occur with polymer materials which have already been heated in the cutter compactor up to the vicinity of their melting range. If overfilling of the intake region occurs here, the material melts and intake is impaired.
Problems are also encountered with fibrous materials that are mostly orientated and linear, with a certain amount of longitudinal elongation and low thickness or stiffness, for example plastics foils cut into strips. A main reason for this is that the elongate material is retained at the outflow end of the intake aperture of the screw, where one end of the strip protrudes into the receiver and the other end protrudes into the intake region. Since the mixing implements and the screw are moving in the same sense or exert the same conveying-direction component and pressure component on the material, both ends of the strip are subjected to tension and pressure in the same direction, and release of the strip becomes impossible. This in turn leads to accumulation of the material in the said region, to a narrowing of the cross section of the intake aperture, and to poorer intake performance and, as a further consequence, to reduced throughput. The increased feed pressure in this region can moreover cause melting, and this in turn causes the problems mentioned in the introduction.
Various extruders have been attached to co-rotating cutter compacters of this type, and all of the results here have in principle been acceptable and interesting. However, the applicant has initiated comprehensive studies with the aim of achieving even more improvement of the entire system.